Nuclear magnetic relaxation methods offer a variety of opportunities for characterizing the molecular dynamics in confined environments. Systems of interest are high surface area materials including biological tissues, chromatographic supports, heterogeneous catalytic materials and natural porous materials such as clay minerals and rocks.
Nuclear magnetic relaxation dispersion (NMRD) consists in measuring the observables of relaxation as a function of the magnetic field. It enlarges drastically the timescale and lengthscale of observation of the molecular dynamics especially in porous media.
NMR has been a common laboratory technique for over forty years and has become an important tool in formation evaluation. General background of NMR well logging can be found, for example, in U.S. Pat. No. 5,023,551 to Kleinberg et al., which is assigned to the same assignee as the present invention and herein incorporated by reference in its entirety.
NMR relies upon the fact that the nuclei of many chemical elements have intrinsic angular momentum (“spin”) and a magnetic moment. In an externally applied static magnetic field, the spins of nuclei align themselves along the direction of the static field. This equilibrium situation can be disturbed by a pulse of an oscillating magnetic field (e. g., a radio frequency (rf) pulse) that tips the spins away from the static field direction. The angle through which the spins are tipped is given by θ=γ*B1tP, where γ is the gyromagnetic ratio, B1 is the linearly polarized oscillating field strength, and tP, is the duration of the pulse. Tipping pulses of 90 and 180 degrees are most common.
After tipping, two things occur simultaneously. First, the spins precess around the direction of the static field at the Larmor frequency, given by ω0=γ*B0, where B0 is the strength of the static field and γ is the gyromagnetic ratio. For hydrogen nuclei, γ/2Π equals 4258 Hz/Gauss. Secondly, the spins return to the equilibrium direction according to a decay time, T1, which is known as the spin-lattice or longitudinal relaxation time.
Also associated with the spin of molecular nuclei is a second relaxation time, T2, called the spin-spin or transverse relaxation time. At the end of a 90-degree tipping pulse, all the spins are pointed in a common direction perpendicular, or transverse, to the static field, and precess at the Larmor frequency. However, due to small fluctuations in the static field induced by other spins or paramagnetic impurities, the spins precess at slightly different frequencies, so that the transverse magnetization dephases with a relaxation time constant T2.
Most NMR logging operations measure the spin-lattice (longitudinal) relaxation times (T1) and/or spin-spin (transverse) relaxation times (T2) of hydrogen nuclei. In addition, some NMR logging tools may provide a ratio of T1/T2 directly, and other NMR tools can provide diffusion constants (D) and combined D-T2 plots.
Various pulse sequences are available for measuring the NMR relaxation times. For example, T1 relaxation may be measured using an inversion-recovery or a simple spin-echo pulse sequence or any of their derivatives. The T2 relaxation is often measured from a train of spin-echoes that are generated with a series of pulses such as the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence or some variant of this. The CPMG pulse sequence is well known in the art. (See Meiboom, S., Gill, D., 1958, “Modified Spin Echo Method for Measuring Nuclear Relaxation Times,” Review of Scientific Instruments, 29, 688-91).
Wettability of oil/water liquids mixtures measured in porous rocks is one of the most critical parameters for oil recovery with porosity and permeability. Roughly, wettability is the ability of a fluid to spread over or “wet” a solid surface. It influences saturation, pore distribution and flow of fluids in porous materials. Nowadays, wettability is mainly measured by macroscopic measurements such as contact angles and capillary pressure curves (Amott and USBM methods).
Methods to determine the wettability of liquids in a porous media are described in various publications and patents. Among those patents are the co-owned U.S. Pat. No. 6,765,380 to Freedman et al., the co-owned U.S. Pat. No. 6,883,702 to Hurliman et al., and the published U.S. patent application 2006/0132131 to Fleury et al.
Studies relating to the frequency dispersion of the spin-lattice relaxation rate 1/T1 can be for example found in the references:                J.-P. Korb, M. Whaley-Hodges and R. G. Bryant, Phys. Rev. E, 56, 2, 1934-1945, (1997);        J.-P. Korb, M. Whaley-Hodges, Th. Gobron and R. G. Bryant, Phys. Rev. E, 60, 3, 3097-3106, (1999);        S. Godefroy, J.-P. Korb, M. Fleury and R. G. Bryant, Physical Review E, 64, 021605, (2001);        S. Godefroy, M. Fleury, F. Deflandre and J.-P. Korb, J. Phys. Chem. B, 106, 11183-11190, (2002) ; and        J.-P. Korb, G. Diakova, R. G. Bryant, J. Chem. Phys. 124, 134910 (2006).        
In the light of the prior art, it is an object of the invention to provide alternative methods for determining wettability and other parameters of a sample of a porous media. More specifically, it is seen as an object of the present invention to provide a quantitative in situ method for determining directly the wettability of liquids and other parameters of a sample of a porous media.